Reinforced Concrete Residential Foundations

[jimbo2]

I will try to keep this as short as I can.

A few weeks ago I asked some questions and received valuable feedback regarding the consolidation of soft clay due to lowered water tables, and the possibility this may contribute to the numerous cases of residential foundation distress in a certain area of the Northeast. Since then I have met with the Town Building Commissioner to determine whether the Town had requested or obtained any data or information regarding water table fluctuations (none), consolidation test data (none), sensitivity (none), etc. What they do have from the design professionals here is limited to Atterberg limits, moisture content and expansive index test results, and no detailed analysis or concrete design computations.

With regard to the possibility that lowering the water table may have resulted in the observed foundation problems, I now believe that while this may be a contributing factor, I would expect to see a more widespread and uniform distribution of the problem, and not only in residential foundations. Also, I believe that lowering the water table does not necessarily result in a proportional increase in effective sress at depth, especially if an underlying confined water bearing zone is present. We just don't know at this time.

While in the Commissioners office I viewed a few photographs of foundation failures, many times consisting of diagonal cracking and a broken cantilever shape that appears to confirm what Robert168 rightfully pointed out, it is differential settlement that we seem to be experiencing here.

In March, 2003, the Town began requiring builders to place vertical reinforcing steel in accordance with the guidelines included in the State building code, plus 2 horizontal rows of 2-#4 bar in the wall and one horizontal row of 2-#4 bar in the footing. Since the requirement for horizontal steel is more stringent than the State Code, the Town required permission from the State Code Commission to enforce it. Earlier this week, the Town and the Building Association (who is fighting the default use of this more stringent requirement) argued their case in front of the Commission, and the Town lost.

Now, I have gotten to the point where I want to see more than one borehole at a building site to check for variability in the thickness and elevation of the soft clay, including "undisturbed" sampling for laboratory testing of the their compressive properties. I will also ask that a small pourous tube piezometer be placed in the completed borehole for continuing water table measurements. I have also spoken with four different architects and engineers who are preparing these home designs (some costing over $700,000 to build). I am told the design footing pressures (using the Town required minimum 24-inch wide footer) for one of these homes varies between 1700 to 1000 psf along the length of the footer. There is no attempt by the engineers I have spoken with to balance these pressures to help address the possibilty of differential settlement. In fact, these folks are specifying reinforcing steel only because they have to and without computing shear, tensile or bending stress etc. One engineer I spoke with does not have a copy of the ACI Code. I am somewhat taken back by the lack of engineering being practiced here.

My goal now, is to particiapte in developing, or finding an existing, methodology that begins with the collection of the proper amount and type of subsurface information, and incorporates this with site specific residential design data that will establish the proper (plain or reinforced) concrete foundation design. I am imagining a bracketing of the probable soil reactions given the initial design footing pressures, to develop shear and bending diagrams useful in fundamental reinforced concrete design.

I beleive this is not a simple task, but also cannot imagine this has not been done before. I am surprised the people I have sploken with have not done this type of thing, and am having some difficulty getting other local engineers on board with this idea. Am I on the wrong track? Any ideas?

 

[PSlem]

I routinely do the grade beam calcs for piling. At 7' spacing the steel is actually dictated by temperature steel requirements and the shear is less than half so stirrups are not required. This is for 20" deep footings. For basement walls I would not think it would be a concern. I can see slab cracking. Where do walls crack? Near the corners? Maybe not continuous reinforcing there. I would think efforts would be better spent on a basement in adding a foam blocking beside the wall as a bondbreaker and to fill the space over the exposed footing.

 

[jimbo2]

PSlem - thanx for the quick response. I'm not sure I understand the purpose and goal of the foam blocking. Is this to reduce soil backfill loading over the footer? If so, how would this help address the potential for differential settlement?

I know some foundations are designed as pile and grade beam structures; however, the hard material here is usually over 40 or 50 feet deep, and a seven foot spacing on the piles would seem to be much more expensive than constructing a more heavily reinforced concrete foundation to address the range of potential differential loadings and settlements. I do know from one engineer that on foundation repairs he is involved with, they use helical screw piles to underpin the failed foundations, typically placing them about 20 apart. My gut tells me this is way too far apart for a plain concrete foundation, but that is my problem here, the current practice doesn't make sense to me. I experience further confusion when the building commissioner tells me that some repairs consist of screw piles on only one or two walls of the home; thereby appearing to promote future differential settlement problems.

What about a design placing a reinforced concrete foundation on the stiffer stratum nominally above the (variable thickness and separation distance) soft clay? I guess we need to assume either a beam or cantilever configuaration. I continue to look for some rationale approach, given the geotechnical data and the structure loads, to establishing a design loading scenario.

Of the photo's I saw, some cracks were near a corner, others not.

Thank-you again for the input!

 

[PSlem]

The standard method seems to be to place the foundations at a depth below the seasonal variations in moisture content and use a void former (a cardboard box mat) under the slab so it will not rest directly on the soil. Soil investigation will determine requirements. The foam acts as a bondbreaker and provides room for lateral expansion of the soil. I picked up an excellent book about a month ago. Structural foundation design for low rise buildings by Atkinson. I found it by searching the ASUC web site but here's a start

http://www.asuc.org.uk/asuc/publications.html

 

[ishvaaag]

The area in which I practice has been using geotechnical reports for main (medium rise etc) maybe over 20 years and I would say for houses since 10 years or so. Here (Spain) at least in theory all elements are calculated, if a continuous beam foundation, such thing. For small structures it may be feasible to design a stiff (non pile) foundation able to counteract somewhat differential settlement. For all other cases when the firm layer within the ground is very deep all kinds of foundations (including piles) become quite dependent on soil itself respect deformation, since the foundation is not stiff enough to counteract the dishing action under the weight, or other kind of setlement, and the whole foundation is like floating in the mass.

For foundations that the footings in combination with some attached walls are very stiff enough whilst the center is less stiff (not having the wall stiffener) one must expect bigger differential setlement and the triangular cut crack at corners, which appear as a need of compatibility of deformations to take the ground loads in the way actually are being taken.

Hence, proper geotechnical report and sound structural design are true requirements of a good foundation for a building. Without these we only have a risky bet.

Respect geotechnical investigation by public bodies, it is useful to some point; they use here to try to identify conflictive spots and then warn designers where and when they are detected (usually following some problem in some works). However they are pursuing their own goals, and the people tasked with design of buildings, structures and foundations must warrant by their deeds that everything about is properly cared for, so insistence of all necessary requirements being in place before any body is a mandate of good professional behaviour.

In this sense I find quite ridiculous that public associations may fight so sound and modest requirement as to add some steel to some specific types of foundations that have been showing some problems. Of course it will cost a bit more, but the users are those will be paying, and for their good. They would do better in caring in building better, instead of fighting something that will diminish problems for all.

 

[Robert168]

Differential settlement may result from many causes, such as changed thickness of soft layer under influence, different response of soils to loading and moisture, shear failure in foundation soil or differential loading, etc.

Damages or cracks in structures due to differential settlement can be prevented from happening by using 1, soil treatment; 2, reinforced foudations; or 3, a combination of 1 and 2.

Simply reinforcing a foundation without a thorough understanding of the real cause of defferential settlement is at least not cost effective. The problem may happen again if reinforcement is not sufficient under new conditions. If the area in question is big enough, the cause for the defferential settlement may be different from place to place, hence different solution is required on a case to case basis.

 

[dirtsqueezer]

Just an addendum to the discussion. If you haven't already seen this, there is also a test one can perform for consolidation.

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/HISTORICAL/D2435-96.htm?L+mystore+dlss0368

Just a rookie here, but I usually see preloading used in n where settlement is an issue. As far as the commissioner goes, I would definitely more readily trust my gut. Best of luck.

 

[jimbo2]

I appreciate all your comments.

Many wonder why the Building Association would fight placing, what amounts to about $1500 extra for the reinforcing steel, into a foundation for multi-100K homes. The builders say officially that this might eliminate some from qualifying for a mortgage. This is absurd given the clientele here. I wonder if the builders think they are at risk/liable if it becomes obvious that at least some of the failed home foundations should have been reinforced, and/or other soil treatments or soft soil removal should have been considered.

I wonder about just providing a stiff yet ductile reinforced footing, supporting a plain concrete wall. I continued to experiment with loading diagrams on a 12" x 24" footing with two rows (top and bottom) of three #5 bar. The downward loading from the structure can simply be obtined from the architect, but the universe of non-uniform soil pressure/reaction distributions that might be caused by variations in soil properties (once we characterize the soil properties with a proper geotechnical investigation)is proving a bit much for my meager mind.

 

[DenverKev]

Jimbo2,

You talk about a ductile footing. A brick foundation using lime mortar would qualify as a ductile footing. Of course, this was a common way to do it before concrete took over. This was actually a pretty "forgiving" solution to all the potential crack-causing problems.

There might be a lot to learn from the older buildings in the area.

 

[AlohaBob]

Consolidation or contraction causing settlement of large magnitude can be accomodated. Differential settlement is more difficult. The trick is to help the settlements that will occur be uniform. By taking a careful look at the foundation soil pressures and sizing the footings so that they are uniform throughout the entire foundation is the goal. If the soil strata varies across the small length of the building, differential settlement is likely unavoidable. Then perhaps it is wise to use a pile foundation to reach a depth of more uniform soil characteristics.

The two continuous bars at the top and bottom are wise. Minimum detailing requirements in seismic zones require it, so the IBC now should be leading all jurisdictions to observe minimum detailing requirements. Once cracking occurs, the capacity of the foundation to resist differential settlement is essentially void. The visible problems will accellerate at an even worse rate. The minimum detailing steel will help keep the structure serviceable throughout it's expected life cycle.

 

[tree1]

Jimbo,

The design of a structure that will cost at leaat a few hundred thousand at least warrants some real soil testing (ie more than Atteberg Limits and a moisture & density test).

I am not sure about the age of the structures that you are looking at. If there are older structures have they also experienced distress but because of age or type of construction it has not been an issue?

I think you will find with newer, pricier homes you are going to get more cheesecake (stone or ceramic tile floors, more exotic framing designs, larger openings etc.) All these can make for what may have been a reasonable amount of deflection from a structural standpoint a moot point to an owner who doesn't want cracks.

I think you first need to figure out what is the nature of the soft clay deposit (uniform thickness or highly variable) and develop some consolidation data and maybe some expansion tests. This will be the hard part as no developer/owner wants to be the first on the block to ante up money for "excessive testing". Unfortunately, I think you often will find that you need to have problems arise in a structure or area that somebody will pony up the money to do an analysis that probably should have been done inthe first place.

I se you have discounted regional groundwater lowering. If you still want to check out look for municipality/utility repairs on lines, especially those that are gradient sensitive.

 

[PSlem]

I've been boxing books today to take to the office and ran into an article in the Nov. '02 issue of the Journal of Light Construction magazine on Trouble-Free Foundations for Expansive Soils. It says many states, such as Colorado entitle the buyer of a lot in a developed subdivision to a copy of the soils report for the original developer. if you have problem soils the choices are an expensive and heavily reinforced waffle grid slab used without basement where caissons would be too deep and costly or isolation, which involves grade beams with void formers under them atop caissons and preferably a structural wood floor. If a concrete slab is used, interior walls are held two inches off the floor with 60d nails through the top plate into the base plate nailed to the slab for location and nailed to overhead joists. A baseboard hides the gap.